I have prepared some UHTCs through hot pressing at 1700, 1850 and 2000 °C. Open pores were dominant type of porosity in the samples hot pressed at 1700 °C, but the type changes completely to closed pores at higher sintering temperatures. How can I rationalize this observation?
May I first suggest running a DSC on your samples, if it goes that high in temperature. That would provide melting/sintering information in addition to gas displacement information. In addition, if your samples are of uniform thickness, some kind of opacity or light scattering information would also be useful. Finally, a density measurement would be instructive.
Then we can turn our attention to the most likely explanation: sufficient fusing/melting to eliminate some of the voids. The most likely steps involve first the sealing off of open cell pores as sintering proceeds to the actual fusing or melting process. The longer the sample is heated (of the higher the temperature), the more elimination of voids that will occur. if this hypothesis is correct, you will also note a decline in opacity/light scattering, an increase in density, and an increase in stiffness/brittleness,
In addition, DSC would provide much confirmatory information on various temperature-based transitions, such as melting, fusing, and pore elimination. Melting would be the strongest response, while the others may be more subtle.
If you're interested in a more mild model to study, may I suggest a p-styrene latex of particle size similar to that of your ceramic. Once dried at room temperature, the fragile sample will progressively fuse as the temperature rises above ~70 deg C up to about 200 C.
Regards
I endorse both Floyd's and Konegger's reasoning about the state of your material at the said temperatures, where pores usually get clogged due to surface diffusion of particles and sintering. Although your material composition is not conceretly presented, it has to be characterized if there is any influence of temperature due to change in melting point in the temeparture window of 300 oC.
Hi, first you shold clarify what kind of pores are considered, I mean what is size of the porosity you are talking about. Depending on size, you may observe your material usin micropscopic techniques (wide range) or adsorptive methods. For nanoporosity adsorption of gases e.g. nitrogen at 77K gives you much information concerning porosity.
Jacek.
Are you able to determine the specific weight in g/cm³ of the two material variants? Sometimes simple methods may help.
K.F.
I agree with all previous remarks.
I just want to mention that your result is very common when you increase the sintering temperature (read general books on sintering). Even the exact mechanism remains unclear (solid-state diffusion, liquid phase formation, etc.) because of the lack of information you provide (DSC and/or ATG can help you), porosity closing is certainly due to densification at high temperature.
At 1700°C you measure both open and closed porosity because the mean relative density of your composite should be inferior to 95% (liquid penetration inside your sample through open porosity-except in closed porosity).
At higher temperatures, you only measure closed porosity because your relative density should become higher than 95-97%.
The decreasing of porosity amount with SiC addition should be explained by a better sinterability of your composite material (i.e. a decreasing of sintering temperature), which could be explained by a better initial compacity, by the formation of a solid-solution or liquid phase during sintering or maybe simply because of a lower refractoryness of SiC compared to your matrix (B4C for example).
Hope this helps !
R.B.
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